Dorsal fuel tanks - more weight and less pay load for weapons. You would need a new engine.Dorsal fuel tanks allow for increased range. Larger nose cone allows for better radar, extra space occupied previously by the single inlet can be used for better avionics and computers, having MFDs allows for ground attacks using precision guided munition.
Plus all of this could be done for cheap, imagine the capability you're getting compared to the cost.
New avionics and radar is not cheap either. You would need extensive testing as these airframes are 30 years old.
The money and time spent doing this for a fleet of around 50 planes. Why not just buy another squadron of Block IIIs?